In semiconductor manufacturing, several processes exist for adding a layer of material to a semiconductor substrate, including transferring a layer of material from one semiconductor substrate to another. Such processes include methods for forming silicon-on-insulator (“SOI”) wafers, semiconductor-metal-on-insulator (“SMOI”) wafers, and silicon-on-polycrystalline-aluminum-nitride (“SOPAN”) wafers. For example, FIGS. 1A-1D are partially schematic cross-sectional views illustrating a semiconductor assembly in a prior art method for transferring a silicon material from one substrate to another. FIG. 1A illustrates a first substrate 100 including a base material 104 and an oxide material 102 on the base material 104. As shown in FIG. 1B, a second substrate 120 is then positioned on the first substrate 100 for bonding (as indicated by arrow A). The second substrate 120 also includes a silicon material 124 and an oxide material 122 on the silicon material 124. The second substrate 120 is positioned on the first substrate 100 such that the first substrate oxide material 102 contacts the second substrate oxide material 122 and forms an oxide-oxide bond. The silicon material 124 has a first portion 124a and a second portion 124b delineated by an exfoliation material 130 at a selected distance below a downwardly facing surface of the first portion 124a. The exfoliation material 130 can be, for example, an implanted region of hydrogen, boron, and/or other exfoliation agents.
FIG. 1C illustrates the semiconductor assembly formed by bonding the first substrate 100 to the second substrate 120. Once the substrates 100, 120 are bonded, the first portion 124a of the semiconductor material 124 is removed from the second portion 124b by heating the assembly such that the exfoliation material 130 cleaves the silicon material 124. The second portion 124b remains attached to the first substrate 100, as shown in FIG. 1D, and has a desired thickness for forming semiconductor components in and/or on the second portion 124b. The first portion 124a of the silicon material 124 can be recycled to supply additional thicknesses of silicon material to other first substrates.
One challenge of transferring silicon materials from one substrate to another is that poor bonding between the first and second substrates can greatly affect the yield and cost of the process. In transfer processes, “bonding” generally includes adhering two mirror-polished semiconductor substrates to each other without the application of any macroscopic adhesive layer or external force. During and/or after the layer transfer process, poor bonding can cause voids, islands or other defects between the two bonded substrate surfaces. For example, the material properties of certain materials can result in poor bonding, such as silicon with a metal of SMOI or a poly-aluminum nitride surface with silicon of SOPAN.
Conventional bonding processes also include applying an external mechanical force (e.g., a weight or a compressive force) to the first and second substrates for a period of time. In addition to adding time, adding cost, and reducing throughput, bonding processes that use an external force suffer from several drawbacks. First, the downward force applied on the second substrate is not distributed uniformly and can cause defects in the substrate or even break one or both substrates. Second, because the force is not distributed uniformly, the magnitude of the applied mechanical force is limited to the maximum allowed force in the area with the highest force concentration, which means that other areas of the substrate do not experience the maximum force. Third, the use of an external mechanical force can contaminate the semiconductor assembly. Last, some semiconductor devices can have large depressions in the under-layer topography because of dishing from prior chemical-mechanical planarization processing making it difficult to bond the oxide layers to each other.